Wear-resistant alloys on the basis of iron are widely used. In this connection, the resistance to wear is achieved from the hardness of the martensitic metal matrix and the content of hard carbides, nitrides or borides of the elements chromium, tungsten, molybdenum, vanadium, niobium or titanium. This group includes cold work steel and high-speed tool steels, as well as white cast iron and hardfacing alloys.
Powder-metallurgical steel alloys were developed when striving for fine carbides, their homogenous distribution and high contents, in order to improve the resistance to wear. The starting powder of these materials is an alloyed powder that is created by atomizing a melt. Normally powders of this type are filled into thin sheet metal capsules that are compacted into a dense body after the evacuation and seal welding in special autoclaves, using the hot isostatic pressing (HIP) technique at a temperature below the melting point and at an isostatic gas pressure of up to 2,000 bar. By means of subsequent hot working (forging or rolling), the compacted capsules are reworked into semi-finished products of tool steel that are available on the market in various dimensions. Generally tools are produced from these semi-finished products, whereby these tools obtain their service hardness by means of a heat treatment known as hardening. The hardening consists of austenitizing and cooling at such a speed that predominantly a hard martensitic structure is formed. As the wall thickness of the workpiece increases, the cooling speed needed for this is no longer reached in the core and the high degree of hardness of the martensite can be regulated only down to a certain depth in the workpiece. This is called the effective hardening depth. In this case, the core is not through-hardened.
A multitude of powder compositions for wear-resistant materials are known, but these generally are not sufficient for thick-walled composite parts as far as their through-hardening characteristics are concerned. By way of example in this connection, mention is made of a steel matrix hard material composite material, disclosed in DE 3508982, as well as a powder-metallurgically produced steel product with a high vanadium-carbide content, as described in DE 2937724 and EP 0515018.
HIP technology can be used for more than just the production of semi-finished products made of powder-metallurgically produced steel; it is also suitable for applying a layer produced from powder with a thickness in the mm to cm range onto an economical, usually resistant steel substrate. This technology, known as HIP cladding, is being more and more widely used for the production of components that are subject to heavy wear and that are used in processing technology and polymer processing. Some examples of substances used in this case as wear-resistant layer substances are atomized steel powder, to which hard material powder is additionally added in some cases, with a view to a high level of wear-resistance. In this way, today even workpieces with extremely wear-resistant layers can be provided that greatly surpass, as far as the life cycle is concerned, the conventional wearing components not produced in the powder-metallurgical manner. New HIP systems are being made for larger and larger components, which consequently also have larger and larger wall thicknesses. This leads to the development of the problem of insufficient hardening for the heat treatment of the larger-walled composite parts after the HIP step.
The objective of this heat treatment is the martensitic through-hardening of the layer substance, which, in operation, is largely consumed by wear and which consequently must be through-hardened. Because of the high risk of cracks and distortions in alloys containing hard material and the sudden cooling in water or oil, these cooling media are ruled out, particularly in the case of thick wall thicknesses, because of the associated large thermal tensions. For this reason, there is a demand for layer substances that can be converted to the martensite phase that is needed for a high level of wear-resistance, even in the case of the slow cooling of large composite components, e.g., in the air, vacuum ovens with nitrogen pressure <6 bar or in the HIP system. The steel powders known today are not suitable for this purpose, because they have been optimised for semi-finished products and workpieces with smaller wall thicknesses.